The present invention relates to an anti-saturation circuit for preventing saturation in a communications receiver despite the presence of a strong interfering signal.
Saturation in this context refers to a condition in which an amplifier or other circuit element becomes overloaded and cannot respond linearly to an input signal. Communications receivers generally have an automatic-gain-control facility that avoids this condition by keeping the level of the desired signal below the saturation level, but when an interfering signal stronger than the desired signal is present, saturation may occur despite automatic gain control. Depending on the frequency relationship between the desired signal and interfering signal, the result will be intermodulation interference, defined in the IEEE Standard Dictionary of Electrical and Electronics Terms as "modulation products attributable to the components of a complex wave that on injection into a nonlinear circuit produce interference on the desired signal."
One system in which intermodulation causes problems is the North American code-division multiple-access (CDMA) digital cellular telephone system, which receives strong interfering signals from the analog Advanced Mobile Phone System (AMPS). CDMA receivers are designed for linear operation of their circuit elements up to a specified level of interference, but interference from nearby AMPS stations can easily exceed the specified level. If saturation occurs, the resulting intermodulation products impair the sensitivity of the receiver, making further anti-saturation measures necessary.
A simple anti-saturation scheme employed in conventional CDMA receivers reduces the gain of the low-noise amplifier that amplifies the radio-frequency signal received from the antenna. The low-noise amplifier operates with a standard fixed gain until the strength of the desired signal reaches a certain threshold; then the gain is reduced by a fixed amount. To avoid oscillation between the standard and reduced gain values, a pair of thresholds is employed. The standard gain is used below the lower threshold, the reduced gain is used above the upper threshold, and the standard or reduced gain is left unchanged between the two thresholds. The gain control characteristic is thus a step-function with hysteresis.
This simple system is unsatisfactory for several reasons. A first reason is that, to allow a margin for fading, the lower threshold must be set conservatively, at a relatively high value, leaving a large range of signal strengths over which no anti-saturation measures are taken. A second reason is that a gain reduction large enough to have an adequate anti-saturation effect is also large enough to impair the receiver sensitivity in the region just above the lower threshold. A third reason is that the abrupt switching of the low-noise-amplifier gain between the standard and reduced values tends to disrupt automatic gain control, even if a compensatory signal is applied to the automatic-gain-control amplifier. In CDMA systems, the strength of the received signal is used for both receiving and transmitting gain control, so both forms of automatic gain control are disrupted. Further details will be given later.